Natural selection gave a freediving people in Southeast Asia bigger spleens

Laverne Higgins
April 20, 2018

Unless you have this rare gene mutation - a "surprise finding" identified by scientists in a new study - chances are you can only hold your breath for seconds, or a few minutes at most. The Bajau, renowned for their breath-holding and prolonged free-diving abilities, possess gene variants that result in larger spleens-and larger reservoirs of oxygenated red blood cells.

Findings of a new study now reveal that their incredible ability to dive deeper underwater and go longer without oxygen is a genetic adaptation.

Specifically, the genetic mutation researchers found that had spread through the Bajau population "upregulates" thyroid hormone - a hormone that, in mice at least, has been linked to bigger spleens. Ilardo, who undertook the work as part of her doctorate, was warned to not take the risk, since other diving adaptations had been shown to have no genetic foundation.

The scans showed that the spleens of the Bajau, whether from divers or nondivers, are 50% larger than those of the Saluan.

The team also discovered that members of the Bajau have a gene called PDE10A which the Saluan do not.

This gene is also though to control the thyroid gland and its principal hormone Thyroxine or T4. Of these, one site on a gene known as PDE10A was found to be correlated with the Bajau's larger spleen size, even after accounting for confounding factors like age, sex, and height.

Physiologists know that most mammals-including humans-undergo a "diving response" when their faces hit cold water.

It's possible that the Bajau's way of life has been ongoing for more than 1,000 years, researchers said, citing linguistic analysis. "The only trait previously studied is the superior underwater vision of Thai Sea Nomad children, however this was shown to be a plastic response to training, and was replicable in a European cohort".

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Ilardo is the first author of a study ("Physiological and Genetic Adaptations to Diving in Sea Nomads") that appeared April 19 in the journal Cell.

Astonishingly, these deep dives are performed only with a wooden mask or goggles and a weight belt.

At first glance, the spleen doesn't seem a likely organ to help us hold our breath. While competitive divers can train to boost their lung capacity or increase their red blood cell count, the current study offers fresh evidence of the potential for humans to adapt genetically to a lack of oxygen and support a lifestyle centered on diving for food. She traveled to seaside villages in the Central Sulawesi peninsula near Indonesia, where both the Bajau and an unrelated population, the Saluan people, live, and recruited 43 Bajau and 33 Saluan individuals to participate.

The Bajau, a group of "Sea Nomads" who live on houseboats traveling the waters of Southeast Asia, rely on fantastic diving prowess to survive.

"For possibly thousands of years, [they] have been living on house boats, travelling from place to place in the waters of South-East Asia and visiting land only occasionally". A chronicler of one of Ferdinand Magellan's voyages recorded their unusual lifestyle in 1521. "I basically just showed up at the house of the chief of the village, this freaky, foreign girl with an ultrasound machine asking about spleens", she recalled. "A lot of them are threatened and this is not just a loss culturally and linguistically, but for genetics, medicine, and sciences in general. And we do have a trip planned to return to the community to explain the results to them".

Dr. Ilardo stated that Bajau people were like "super-humans" and that had "extraordinary" capabilities but also stated that natural selection is much more powerful than realized and should be given more credit.

As humans, we sometimes like to think we're immune to natural selection, but this study shows that's simply not the case.

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